Anti Matter – The Dark Twin

What Is Anti Matter?

We see things around us made of regular matter all the time. Your clothes, your books, a guitar, a football – everything can essentially be broken down into fundamental constituents – protons, neutrons and electrons, along with other quirky particles with short lifetimes. And everything we study in physics depends on interaction of all this “regular” matter. But this matter has a twin too – which we don’t see in day to day life. It looks identical to matter in, and when it comes in contact with matter, they annihilate each other. This evil twin has a name – Anti Matter.

Well, not necessarily evil, but anti-matter does do the job of destroying our regular matter and converting it into Energy. While such a thing would be considered blasphemous by Newtonian scientists, we know that Einstein took care of this. He showed that matter and energy can indeed be converted from one form to another, by formulating the most famous equation in the history of physics, E = mc2.

Anti-matter is often said to be regular matter traveling backwards in Time. While it sounds a bit weird, the first discovery of the positron made Carl Anderson, the first person to observe anti-matter, also think that is was just an electron traveling backwards in time. In Feynman Diagrams too, anti-particles are represented as particles traveling backwards in time. Anti-matter is identical to matter in almost every other sense – they have the same mass, same shape, and undergo kinda similar decays and interactions. The only difference is in the charge, or to be more technically correct, the quantum numbers of the particles.

Every particle has a set of numbers that defines all properties of the particle, and they are called the Quantum Numbers of the particle. For instance, every particle has a charge, a spin, and something called an Iso spin. Other than that, leptons have a Lepton Number that is conserved in reactions, baryons have a Baryon Number, and so on. The quantum numbers for a particle are just the negative of the quantum numbers of the anti-particle.

Let us take an electron, for instance. The anti-particle of the electron is an anti-electron, or as historically named, a positron. Since an electron has a charge of -e, the positron has a charge of +e. If the electron spin is -1/2, the positron spin will be +1/2. This applies to all properties of the particle, except mass, as there is no negative mass possible.

Anti-particles were first theorized by Dirac, after formulation of the Dirac Relativistic Wave equation for spin – 1/2 particles. Dirac observed that the solution to this wave equation was in the form of a 4 – component spinor, and hence had 4 possible solutions. Of these, 2 solutions had a negative energy state.

The Dirac Equation in terms of the 4 – component spinor.

Rather than seeing it as a flaw in the theory, Dirac hypothesized that these negative energy states corresponded to anti-particles. Dirac explained his theory for electrons and proposed the Electron Sea – an infinite sea of electrons with negative energies. This was an essential addition. Without the sea, if a negative energy state was stable, then every electron would keep falling into lower energy states forever. Since electrons obey the Pauli Exclusion Principle, they cannot be in the same state as the electrons in the sea. If, in case, an electron from the sea gains some energy and becomes a positive energy electron, there is now a hole left where the electron was. This hole was the supposed anti-particle of the electron – the positron.

While not entirely accurate, this was the first consistent model of existence of anti-matter. In 1932, Carl Anderson found the first ever anti-particle – the positron. Dirac’s hypothesis, was once and for all, proven to be accurate.

The trail initially looked like an electron traveling backwards in time, but Carl Anderson proposed it to be a positron.

Today, we can produce anti-matter in large quantities. Anti-matter is produced by a process called Pair Production. Einstein’s equation shows that energy can be converted into mass, but it has to be in the form of a matter – anti-matter pair. These are produced at very high energies in particle accelerators, and then can be trapped and studied.

But why particle accelerators? Why don’t we see anti-matter around us in the Universe?

This is one of the most challenging questions in research today – The Baryon Asymmetry. If the Universe was born out of nothing, then matter and anti-matter must have been produced in the same amount. If this happened, they would instantly annihilate each other, releasing tremendous energy. This would mean that matter would simply not exist, neither would stars, neither would galaxies, neither would planets, and, well, us.

But all of this does exist. Somehow, we observe that matter was apparently slightly more abundant in the earlier Universe. So while all the anti-matter got annihilated, some regular matter still remained. Why the Universe supposedly favour matter over anti-matter is a question we don’t know the answer to yet.

What we do know is that anti-matter exists, and thanks to the work of Dirac and his subsequent researchers, we know how it behaves. We can only hope that in the subsequent years, this last piece of the anti-matter puzzle gets resolved too.